For several decades now, all experimental data on ion transport in epithelial tissues were interpreted on the assumption that the tissue can be represented as the "three compartment model". It is now clear that this concept is no longer tenable. We have developed a carefully considered multicompartment model which is in better agreement with anatomical and physiological facts. We are now able to present a complete, numerical description of the kinetic events as they occur under normal physiological conditions, and also under a variety of pharmacological conditions, such as treatment of transporting epithelia with amiloride, or glycosides. We wish to extend our modeling work in two directions: 1) We would like to consider problems of volume regulation. Thus far we have always assumed that the volumes of the multicompartment model membrane remain constant. 2) We wish to include problems of ion coupling. Thus far we have only considered the flux of Na ion. Questions of interest are: What are the roles of K ions and Cl ions? Solutions of these problems will be sought by the application of the Continuous System Modeling Program (CSMP).